Ink-receptor sheet for use as a recording material

ABSTRACT

A recording sheet for use on a digital press includes either insoluble aluminium trihydrate in the base paper or magnesium sulphate at the surface. According to a second aspect of the invention, the recording sheet includes a paper substrate having a surface treatment including a water soluble cationic substance and a water soluble binder substance.

[0001] The present invention relates to a recording medium and inparticular, but not exclusively, to a plain paper recording medium foruse with the Indigo™ digital printing press.

[0002] The Indigo digital press made by Indigo NV requires special papersurfaces to enable complete toner/ink transfer from printing blanket topaper. Such complete transfer is essential to allow variable informationto be printed on successive sheets.

[0003] It is known that commercially available pigment coated printingpapers satisfy the requirement of 100% toner transfer, however foraesthetic and cost reasons printers wish to use papers of uncoatedappearance. For instance: a translucent or semi-translucent paper wouldhave its translucency diminished by pigment coating; coated papers areoften glossy; the surface texture of an uncoated paper feels morenatural.

[0004] Two processes have been developed to treat paper to render itsuitable for the Indigo press. One developed by Indigo NV and describedin International patent application WO96/06384 is commercially known asthe Sapphire treatment and involves treating the paper with polyethyleneimine. That developed by Arjo Wiggins and described in EP 0879917 A2involves use of a surface treatment that includes an aluminate salt or asalt of a weak acid/strong base to render the surface alkaline.

[0005] It is an object of the present invention to provide a paper or arange of papers of uncoated appearance that are suitable for use on anIndigo press and also in other digital and non-digital processes, andwhich also have an increased shelf life.

[0006] According to one aspect of the invention there is provided arecording sheet for use on a digital press, the recording sheetincluding a paper substrate containing an insoluble mineral fillerincluding a Lewis acid. The recording sheet may, for example, containaluminium trihydrate (Al(OH)₃). The recording medium may be an opaquepaper including a surface treatment, for example polyvinyl pyrrolidoneand magnesium sulphate or starch plus polyvinyl alcohol (PVOH).

[0007] Advantageously, the paper substrate contains between 50 and 400,preferably between 100 and 300, more preferably approximately 200 partsdry weight of aluminium trihydrate to 800 parts dry weight of pulp.

[0008] Advantageously, the recording sheet has a surface treatmentincluding magnesium sulphate and polyvinyl pyrrolidone. The surfacetreatment may be applied to the paper at a rate of 2 to 4 g/m².

[0009] Advantageously, the recording sheet has a surface treatmentincluding starch and polyvinyl alcohol. The surface treatment mayinclude an optical brightening agent. The surface treatment may beapplied to the paper at a rate of 1 to 2 g/m².

[0010] Advantageously, the recording sheet has a surface treatmentincluding a soluble or insoluble metal from Groups II and III or theTransition Metals of the Periodic Table.

[0011] Advantageously, the recording sheet is substantially opaque.

[0012] According to another aspect of the invention there is provided amethod of printing on a recording sheet using a digital printing press,characterised in that the recording sheet is as defined in any one ofthe preceding paragraphs.

[0013] According to another aspect of the invention there is provided amethod of manufacturing a recording sheet for use on a digital press,the method including making up a slurry in water containing paper pulpand aluminium trihydrate, and forming the slurry into a web of paper ona paper machine.

[0014] Advantageously, the slurry contains between 50 and 400,preferably between 100 and 300, more preferably approximately 200 partsdry weight of aluminium trihydrate to 800 parts dry weight of pulp.

[0015] Advantageously, the method includes treating the surface of thepaper with a surface treatment including magnesium sulphate andpolyvinyl pyrrolidone, which surface treatment may be applied to thepaper at a rate of 2 to 4 g/m².

[0016] Advantageously, the method including treating the surface of thepaper with a surface treatment including starch and polyvinyl alcohol,which surface treatment may include an optical brightening agent and maybe applied to the paper at a rate of 1 to 2 g/m².

[0017] Advantageously, the surface treatment including magnesiumsulphate and polyvinyl pyrrolidone and the surface treatment includingstarch and polyvinyl alcohol are applied to the paper as separatetreatments.

[0018] In a preferred embodiment, the invention is characterised by thepresence of insoluble aluminium in the base and/or magnesium sulphate atthe paper surface. The paper of the present invention is intendedprimarily for printing on an Indigo digital printing press. We havefound that the use of an insoluble aluminium filler in the base makesthe paper suitable for use in the Indigo press without the need for anyspecial surface treatment. The paper is also suitable for use in theXeikon digital printing press. Further, the paper has universalapplicability, making it suitable for litho, inkjet, laser (mono andcolour) printers and fountain pens. It has the aesthetic appearance ofan uncoated paper, which gives it an advantage over some other Indigoprintable papers that are of coated appearance. The coated papers thatperform well in Indigo are usually designed primarily for litho and donot give good inkjet performance, so are not universal.

[0019] According to another aspect of the present invention there isprovided a recording medium for use on a digital printing press (forexample the Indigo press), the recording medium including a papersubstrate having a surface treatment of a water-soluble cationicsubstance and a water-soluble binder. For example, the recording mediummay comprise a translucent paper with a surface treatment of polyvinylpyrrolidone (PVP) and magnesium sulphate (MgSO₄) with a synthetic sizingagent, for example Baysynthol KSN B. This formula has none of thedrawbacks of available Indigo pre-treatments.

[0020] According to another aspect of the present invention there isprovided a method of manufacturing a recording sheet for use on adigital press, the method including treating the surface of a papersubstrate with a surface treatment including a water soluble cationicsubstance and a water soluble binder substance.

[0021] Embodiments of the invention will now be described by way ofexample.

[0022] We have found that three things work particularly effectively:

[0023] 1. Translucent paper with a surface treatment of polyvinylpyrrolidone+magnesium sulphate+Baysynthol KSN synthetic sizing agent.

[0024] 2. Opaque paper with an insoluble mineral filler of aluminiumtrihydrate (Martifill) and a surface treatment of polyvinylpyrrolidone+MgSO₄.

[0025] 3. Opaque paper with an insoluble mineral filler of aluminiumtrihydrate and a conventional paper surface treatment, e.g. starch pluspolyvinyl alcohol.

[0026] The results of using the above formulae are summarised in thefollowing table: Inkjet inc. Laser pigment Hotmelt (mono & FountainPrintability Indigo Litho inks Inkjet colour) pen Formula 1 yes yes yesyes yes yes Formula 2 yes yes yes yes yes yes Formula 3 yes yes yes yesyes yes

[0027] The surface treatment in formulae 1 and 2 is the same as the onewe claimed for pigment based ink-jet on translucent paper, described inpatent GB 2 301 845 B the contents of which are incorporated byreference herein. Formula 2 also includes aluminium trihydrate as afiller in the base, in addition to the surface treatment.

[0028] It would seem that the Baysynthol is not essential for the Indigoprocess. The suitability of the paper for use in the Indigo processappears to depend on either the presence of either a soluble metalcation (Mg²⁺) at the surface or a Lewis acid filler (Al(OH)₃) in thebase or a slightly “tacky” polymer such as polyvinyl pyrrolidone orpolyvinyl alcohol at the surface of the paper. We suspect that the groupII or III metals are the key. Further investigation of this aspect ofthe invention is required.

[0029] The preferred surface formulation for 1 and 2 is: parts by dryweight e.g. magnesium sulphate 75 polyvinyl pyrrolidone 25 Luviskol K90Styrene copolymer size 0 to 2.5 Baysynthol KSN B Typical applicationweight: 0.5 to 5 g/m²

[0030] The styrene copolymer size is not essential in the formulationfor formula 2 (opaques) but is needed in formula 1 (translucent papers).

[0031] Variants of the formulae, for example as described in GB 2 301845B are possible. For example: cation: polyvalent metals ions of groupsII and III and transition metals of the periodic table cation: polyquaternary amine or other Lewis acids Binder: starch, cationic starch,carboxymethyl cellulose, gelatine, polyvinyl alcohol, polyvinylpyrrolidone, singly or in admixture of 2 or more Base: opacity 20 to98+, grammage 40 to 300 Size: styrene maleic anhydride, polyacrylate,styrene acrylate or other sizes known in the art

[0032] Formulation for Base for 2 and 3:

[0033] Aluminium hydroxide aka trihydrate (e.g. Martifill P2) 18% on dryfibre.

[0034] Possible variants include the internal sizes and differentparticle size of filler. Surface formulation for 3 Starch 200 dry partse.g. oxidised potato - Amylox P45 from Avebe Polyvinyl  25 dry partse.g. gohsenol GL05 from Nippon Gohsei alcohol

[0035] For increased sheet brightness, an optical brightening agent(OBA) may be included in the surface treatment. We have found that it isbeneficial to split the surface treatment into two applications: a)starch plus polyvinyl alcohol plus OBA and then b) magnesium sulphateplus polyvinyl pyrrolidone. The effect of this is to separateapplication of OBA and magnesium sulphate, which can react with eachother and so mutually interfere with the desired function of each.

[0036] An example of a process for making a recording sheet according toa preferred embodiment of the invention will now be described. Apapermaking stock slurry in water was made from 800 parts dry weight ofcommercial bleached chemical pulp and 200 parts dry weight of MartifillP2 aluminium trihydrate (available from Martinswerk GmbH). To this wasadded 2 parts dry weight of alkyl ketene dimer to serve as an internalsizing agent. Retention aids, dyes and optical brightening agents mayalso be added to suit the particular paper making process and thedesired paper aesthetics.

[0037] The stock slurry was formed into a web of paper on a Fourdrinierpaper machine. Other formers would also serve.

[0038] A first surface application was applied to the paper comprisingan aqueous mix of (by dry weight parts) 50 parts Amylox P45 oxidisedpotato starch (available from Avebe b.a.) and 25 parts of Gohsenol GL05polyvinyl alcohol (available from Nippon Gohsei). To this may be addedan optical brightening agent to suit the desired aesthetic properties ofthe paper. This mix was applied to the paper at a rate of 1 g/m² to 2g/m².

[0039] A second surface application was applied to the paper comprisingan aqueous mix of (by dry weight parts) 75 parts of magnesium sulphateand 25 parts of Luviskol K90 polyvinyl pyrrolidone (available from BASFGmbH). This mix was applied to the paper at a rate of 2 g/m² to 4 g/m².

[0040] The finished paper had an attractive, uncoated appearance and wasfound to give good results on the Indigo digital printing press, as wellas in the Xeikon digital printing press and with litho, inkjet, mono andcolour laser printers and fountain pens. The paper therefore hasuniversal applicability.

1. Use of a recording sheet in a digital printing process, said useincluding the steps of: providing a recording sheet that includes apaper substrate containing an insoluble mineral filler, said fillerincluding aluminium trihydrate, loading the recording sheet into adigital printing press, and printing onto the recording sheet using adigital printing process that includes transferring liquid toner ontothe recording sheet by electrophotography.
 2. Use of a recording sheetaccording to claim 1, wherein the paper substrate contains between 50and 400, preferably between 100 and 300, more preferably approximately200 parts dry weight of aluminium trihydrate to 800 parts dry weight ofpulp.
 3. Use of a recording sheet according to claim 1, wherein therecording sheet has a surface treatment including magnesium sulphate andpolyvinyl pyrrolidone.
 4. Use of a recording sheet according to claim 3,wherein the surface treatment including magnesium sulphate and polyvinylpyrrolidone is applied to the paper at a rate of 2 to 4 g/m².
 5. Use ofa recording sheet according to claim 1, wherein the recording sheet hasa surface treatment including starch and polyvinyl alcohol.
 6. Use of arecording sheet according to claim 5, wherein the surface treatmentincluding starch and polyvinyl alcohol includes an optical brighteningagent.
 7. Use of a recording sheet according to claim 5, wherein thesurface treatment including starch and polyvinyl alcohol is applied tothe paper at a rate of 1 to 2 g/m².
 8. Use of a recording sheetaccording to claim 1, wherein the recording sheet has a surfacetreatment including a soluble or insoluble metal from Groups II and IIIor the Transition Metals of the Periodic Table.
 9. Use of a recordingsheet according to claim 1, wherein the recording sheet is substantiallyopaque.
 10. A method of printing on a recording sheet using a digitalprinting press, wherein the recording sheet is as defined in claim 1.11. A method of manufacturing a recording sheet for use in a digitalprinting process on a digital printing press, the method includingmaking up a slurry in water containing paper pulp and aluminiumtrihydrate, and forming the slurry into a web of paper on a papermachine.
 12. A method according to claim 11, wherein the slurry containsbetween 50 and 400 parts, preferably between 100 and 300 parts, morepreferably approximately 200 parts dry weight of aluminium trihydrate to800 parts dry weight of pulp.
 13. A method according to claim 11, themethod including treating the surface of the paper with a surfacetreatment including magnesium sulphate and polyvinyl pyrrolidone.
 14. Amethod according to claim 13, wherein the surface treatment includingmagnesium sulphate and polyvinyl pyrrolidone is applied to the paper ata rate of 2 to 4 g/m².
 15. A method according to claim 11, the methodincluding treating the surface of the paper with a surface treatmentincluding starch and polyvinyl alcohol.
 16. A method according to claim15, wherein the surface treatment including starch and polyvinyl alcoholincludes an optical brightening agent.
 17. A method according to claim15, wherein the surface treatment including starch and polyvinyl alcoholis applied to the paper at a rate of 1 to 2 g/m².
 18. A method accordingto claim 15, wherein a surface treatment including magnesium sulphateand polyvinyl pyrrolidone and a surface treatment including starch andpolyvinyl alcohol are applied to the paper surface as separatetreatments.
 19. A method according to claim 12, the method includingtreating the surface of the paper with a surface treatment including asoluble or insoluble metal from Groups II and III or the TransitionMetals of the Periodic Table.
 20. Use of a recording sheet in a digitalprinting process, said use including the steps of: providing a recordingsheet that includes a paper substrate having a surface treatmentincluding a water soluble cationic substance and a water soluble bindersubstance, loading the recording sheet into a digital printing press,and printing onto the recording sheet using a digital printing processthat includes transferring liquid toner onto the recording sheet byelectrophotography.
 21. Use of a recording sheet according to claim 20,wherein the cationic substance is a soluble polyvalent metal salt. 22.Use of a recording sheet according to claim 21, wherein the cationicsubstance is a salt of a metal from Groups II and III or the TransitionMetals of the Periodic Table.
 23. Use of a recording sheet according toclaim 22, wherein the cationic substance is a salt of a cation selectedfrom the group consisting of Mg²⁺, Ca²⁺, Al³⁺, Zr⁴⁺ and Zn²⁺.
 24. Use ofa recording sheet according to claim 23, wherein the cationic substanceis magnesium sulphate.
 25. Use of a recording sheet according to claim24, wherein the amount of magnesium sulphate applied to the surface ofthe recording sheet is in the range 0.5-3.0 g/m², and preferably 1.0-2.0g/m², and advantageously approximately 1.25-1.75 g/m².
 26. Use of arecording sheet according to claim 20, wherein the cationic substance isa cationic polymer.
 27. Use of a recording sheet according to claim 26,wherein the cationic substance is a poly-quaternary amine.
 28. Use of arecording sheet according to claim 20, wherein the binder substance isselected from a group consisting of polyvinylpyrrolidone, polyvinylalcohol, carboxylated cellulosic polymers, polyacrylic acids,hydroxylated polyacrylates, polyacrylamides, starches and gelatine. 29.Use of a recording sheet according to claim 28, wherein the bindersubstance is selected from a group consisting of carboxyalkyl polymersand hydroxyalkyl polymers, and preferably hydroxymethyl cellulose andhydroxypropyl cellulose, and is more preferably carboxymethyl cellulose.30. Use of a recording sheet according to claim 28, wherein the bindersubstance has a molecular weight in the range 790,000 to 1,350,000. 31.Use of a recording sheet according to claim 28, wherein the bindersubstance is polyvinyl pyrrollidone having a viscosity defined by aK-value of at least 30, and preferably at least 60 and advantageouslyapproximately
 90. 32. Use of a recording sheet according to claim 31,wherein the amount of PVP applied to the surface of the recording sheetis in the range of 0.15-0.75 g/m², and preferably 0.4-0.7 g/m², and isadvantageously approximately 0.5 g/m².
 33. Use of a recording sheetaccording to claim 20, in which the substances are applied to thesurface of the recording sheet as an aqueous solution.
 34. Use of arecording sheet according to claim 1, in which the recording sheet has asubstantially uncoated appearance.
 35. Use of a recording sheetaccording to claim 1, the recording sheet being suitable for use on adigital press, in a lithographic printing process, for laser printing,inkjet printing with dye and pigment based inks and hot melt imaging.36. Use of a recording sheet according to claim 20, wherein therecording sheet is translucent or transparent.
 37. A method ofmanufacturing a recording sheet for use in a digital printing process ona digital printing press, the method including treating the surface of apaper substrate with a surface treatment including a water solublecationic substance and a water soluble binder substance.
 38. A methodaccording to claim 37, wherein the recording sheet is treated byapplying an aqueous solution of the cationic and binder substances tothe surface of the sheet.
 39. A method according to claim 38, whereinthe solution is applied by drawing the semi-manufactured recording sheetthrough a bath of the solution.